1. Field of the Invention
The present invention relates to a focus detection apparatus and, more particularly, to a focus detection apparatus which is suitable for an optical data recording/reproduction apparatus which records and reproduces various data on and from a recording medium, e.g., an optical disk.
2. Related Background Art
A typical focus detection apparatus of this type adopts a so-called astigmatism method described in U.S. Pat. No. 4,358,200. FIG. 1 exemplifies this. A light beam 42 emitted from a light source 41, e.g., a semiconductor laser, is reflected by a first surface 47 of a wedge-shaped plate 43, and forms a spot on a data track 46 of an optical disk 45 through an objective lens 44. A return light beam 49 reflected by the optical disk 45 is incident on the wedge-shaped plate 43 again through the objective lens 44 and is reflected by a second surface 48 thereof to emerge from the first surface 47. Then, the beam 49 is detected by a photosensor 550. The return light beam 49 causes an astigmatism when it passes through the wedge-shaped plate 43, and the shape of the beam spot on the photosensor 50 changes in accordance with the focusing state on the optical disk 45. When this change in shape of the beam spot is detected by four-divided reception surfaces of the photosensor 50, focus detection can be performed.
However, in the conventional apparatus described above, since the light source and the photosensor are arranged at the same side with respect to the wedge-shaped plate, they must be separated a given distance from each other in the direction of the optical axis of the objective lens (i.e., in the direction perpendicular to the surface of the optical disk) so that they can be operated without interfering each other, resulting in a thick apparatus.
In the conventional apparatus, the wedge-shaped plate 43 causes a coma in addition to the astigmatism, and the spot on the photosensor 50 cannot have an accurate circular shape or an elliptic shape, but is deformed as shown in FIGS. 2A, 2B, and 2C. Note that FIG. 2A shows a just-in-focus state, nnd FIGS. 2B and 2C respective show pre-focus and post-focus states. The deformed spot adversely influences focus detection.